This study proposes to test the hypothesis that the age of closure of the lumbosacral vertebral laminae in humans shows significant variation by race and sex and is later than is generally reported in the literature. Such age variation in the development of the lumbosacral skeleton makes it necessary to distinguish between maturationally and anomalously unfused vertebral laminae (i.e., spina bifida occulta). We hypothesize that the percentage of the population with unfused laminae decreases with age over the maturational period, and that this percentage decrease levels off to a limiting value that represents the true incidence of spina bifida occulta for a given race and sex category. We further hypothesize that failure to control for such variation accounts for the discrepant incidence rates of spina bifida occulta reported in the literature. Our main objectives, therefore, are to (1) collect descriptive data on closure of the lumbosacral laminae from birth to 30 years of age for American Black and White males and females; (2) to determine the chronological age at which each race and sex category achieves the limiting value that we hypothesize as its true rate of incidence for spina bifida occulta; (2) to test if there are significant differences in this age between Blacks and Whites and between males and females-- specifically, if Blacks and females are as precocious in vertebral skeletal development as they are in general skeletal development; and (4) to determine if there are significant race and sex differences in incidence of spina bifida occulta so defined.